Every single bodypart I want size or strength on, I MUST train directly, that goes for forearms and neck as well.
I train them once and sometimes twice a week, in part because I use straps (I cannot use chalk in my gym), otherwise it would be once and a MAX of 5 sets of extentions, and same for wrist curls. You can do plate grabs and such like that as well.
I never got too serious about grip training and never got past two standard 25s for a matter of seconds, but serious grip strength people that you can't even tell work out, can wrist curl big big ass weights. Watch your range of motion so you dont cause pain, dont bounce, and dont use weights that kill you. Some guys can wrist curl over 300 pounds but I sure as hell can't, I liked being a raw lifter but it was hurting my heavy lifts so I went straps.